﻿using System;
using System.Collections.Generic;
using System.Linq.Expressions;
using System.Text;

namespace Infrastructure
{
    public static class LinqHelper
    {
        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> one, Expression<Func<T, bool>> another)
        {
            var candidateExpr = Expression.Parameter(typeof(T), "candidate");
            var parameterReplacer = new ParameterReplacer(candidateExpr);
            var left = parameterReplacer.Replace(one.Body);
            var right = parameterReplacer.Replace(another.Body);
            var body = Expression.And(left, right);
            return Expression.Lambda<Func<T, bool>>(body, candidateExpr);
        }
        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> one, Expression<Func<T, bool>> another)
        {
            var candidateExpr = Expression.Parameter(typeof(T), "candidate");
            var parameterReplacer = new ParameterReplacer(candidateExpr);
            var left = parameterReplacer.Replace(one.Body);
            var right = parameterReplacer.Replace(another.Body);
            var body = Expression.Or(left, right);
            return Expression.Lambda<Func<T, bool>>(body, candidateExpr);
        }
        internal class ParameterReplacer : ExpressionVisitor
        {
            public ParameterReplacer(ParameterExpression paramExpr)
            {
                this.ParameterExpression = paramExpr;
            }
            public ParameterExpression ParameterExpression { get; private set; }
            public Expression Replace(Expression expr)
            {
                return this.Visit(expr);
            }
            protected override Expression VisitParameter(ParameterExpression p)
            {
                return this.ParameterExpression;
            }
        }
    }
}

